Method of making dispenser type cathodes



Nov. 28, 1961 o. G. KOPPIUS 3,010,826

METHOD OF MAKING DISPENSER TYPE CATHODES Original Filed March 22. 195] 2 13 1 \10 V I I- a I 7; /5 7 ly 4 J 4 z 4 12 4 INVENTOR #11 OTTO G.KOPPIUS United States Patent Ofifice 3,919,826 Patented Nov. 28, 1961 Original application Mar. 22, 1951, fier. No. 216,972.

Divided and this application Dec. 14, 1955, Ser. No. 555,221

7 Claims. ((31. 75-297) My invention relates to incandescent cathodes and to methods of making the same and isvmore particularly concerned with cathodes of the so-called dispenser type.

The US. Patent 2,543,728 to H. J. Lemmens et al. de scribes a dispenser type cathode in which a reservoir of electron emissive material is located within a cavity of a body of refractory material which body has a porous Wall portion and is tightly closed so that the pores of the porous Wall portion form the largest passageways connecting the electron emissive material to the outside of the cathode. In the specific examples described and illustrated in this patent, the body is tightly closed me chanically by means of tightly fitting surfaces of large area and also by spinning the ends of a metal tube over the ends of a body of refractory material.

The main object of my invention is to provide an improved cathode of the dispenser type and an improved method of making the same.

A further object of my invention is to simplify the construction of and reduce the cost of such cathodes.

Another object is to provide a dispenser type cathode which is particularly suited to mass production.

A further object of my invention is to tightly close the body during the formation thereof and thereby eliminate additional closure operations.

Still further objects and advantages of my invention will appear as the specification progresses.

The cathode according to my invention comprises a compacted composite mass formed of a mass of adherent finely-divided particles of refractory material which has partly or completely imbedded in it a compacted mass of electron emissive material and which has a porous portion whose pores form the largest passageways connecting the electron emissive material to the outside of the cathode. This composite compacted mass is held within a retaining member to which the particles of refractory material adhere.

The retaining member may be in the form of'a tubular or hollow member and I prevent emission from one side of the composite body by using a partition within the member or by giving the refractory material a non-porous portion at that surface which is to be non-emissive. In other cases the retaining member may be a cup-shaped body formed from one piece of metal.

In accordance with the method of my invention, I place within a suitable retaining member a mass of electron emissive material, for instance a pellet, and a quantity of finely-divided refractory material is arranged so that the mass of electron emissive material is partly or completely embedded in the refractory material. I then compress the composite mass under high pressures and heat the compacted mass to cause the particles of refractory material to adhere to each other and to the surface of the retaining member. 7

I prefer to use tungsten as the finely-divided refractory material although good results can be obtained with other refractory metals such as molybdenum, tantalum, zirconium, titanium, vanadium, rhenium, and columbium. As electron emissive materials, it is possible to use various alkaline earth metals or compounds such as barium carbonate, strontium carbonate, calcium carbonate, caesium carbonate and magnesium carbonate. However, I have 2 obtained particularly good results when using a pellet formed of about 50% barium carbonate and about 50% strontium carbonate. I g

In general the retaining member may be made of material adapted to withstand the temperatures occurring during operation of the cathode, such as molybdenum, tantalum, zirconium or, nickel.

In order that my invention may be clearly understood; and readily carried into efiect, I shall describethe same in more detail with reference to the accompanying drawa ing in which:

FIGURE 1 is a sectionized perspective view of a cathode according to my invention,

FIG. 2 is a sectionized perspective view of a cathode according to another embodiment of my invention,

FIG. 3 is a sectionized perspective view of a cathode according to another modification of my invention,

FIG. 4 is a sectionized side view of a mold for carrying out the method of the invention, and

FIG. 5 is a sectionized side view of the mold of FIG- URE 4 when in the closed position.

The cathode illustrated in FIG. 1 comprises a retaining member 1 in the form of a tubular ring of molybdenum. Within member I is a composite compacted mass cornprising a compressed adherent porous mass of 2 of finelydivided tungsten particles in which is completely embedded a compacted mass 3 of electron emissive material, such as barium strontium carbonate. Mass 2 is of porous structure with the pores thereof forming the largest passageways for the egress of the electron emissive material. As a result of my method, which will be described later, the composite compacted mass 2--3 is held in compression by the ring 1 and is firmly held in place by compression and the adherence of the tungsten particles to the inner surface of ring 1. Y

The cathode may be mounted in a discharge tube by means of a suitable metal support which also acts as a conductor for thecurrent to the cathode; The ring 1 which as stated is firmly secured to the composite mass 23 serves to conduct large amounts of current thereto. The cathode may be heated by a filament, by induction or by other methods known in the art.

The cathode illustrated in FIG. 2 is somewhat similar to the cathode shown in FIG. 1 and has the same parts indicated by the same reference numerals. However, in FIGURE 2 the composite compacted mass is firmly secured in the upper cup-shaped portion of a tubular member 4 having apartition 5 of molybdenum which prevents emission into the tube. Partition 5 may form part of tube 4, i.e. made from a single piece, but I prefer to use a separate disc which is held in tube 4 by pressure against the tube and adherence to the mass 2. Within the lower part of tube 4 is located a heating filament 6.

Referring now to FIG. 3, the cathode shown therein is similar to the cathode of FIG. 2 and has similar parts indicated by similar reference numerals. However, in FIG- URE 3 the partition 5 has been omitted and emission downward into tube 4 is prevented by making the lower part 7 of the compacted mass 2 of non-porous structure by a method later to be described.

In general the method according to the invention comprises the pressing and heating of a composite mass consisting of finely-divided refractory material in which is partly or completely embedded a mass of electron emissive material. The pressing and heating takes place while the composite mass is within a retaining member which forms part of the completed cathode. The particle size of the refractory material must be so selected with relation to the pressure'and temperature used in forming the body so as to obtain the desired porosity. For each particle size of refractory metal used, there is a limited pressure range which must be used in forming the cathode, for a given heating temperature. Fine particles require a somewhat lower pressure than coarse particles.

The pressure used must be so selected that, depending upon the particle size and temperature, the desired porosity and proper adherence of the particles to each other and to the retaining member will be obtained. In general the pressures tobe used lie within the range of about 20,000 to 400,000 lbs. per sq. in. The optimum pressure to be used depends upon several factors such as the size of the particles, porosity desired and in some cases the machinability required of the finished body. The temperature used must be so selected that it is not lower than about the temperature at which the cathode is to be operated and is below the value which results in excessive evaporation of the electron emissive material during the heating operation which may take place before the cathode is placed in the tube or while the cathode is being activated in the tube.

The method of the invention will now be described in more detail with regard to cathode structures as shown in FIGS. 2 and 3 and with reference to FIGURES 4 and 5 which show pressing dies for carrying out the method. The dies shown in FIGURES 4 and 5 comprise a mold 8 having a central cavity 9, a central bore 10' and an annular cavity 11. A plunger 12 located in cavity h has an extension 13 passing through bore 10 and extending therefrom a distance equal to the stroke of the plunger.

In carrying out the method, the retainer member 4 of FIGS. 2 or 3 is placed in the mold with its lower end in cavity 11 and the partitioning disc 5 of FIGURE 4, which may be of molybdenum, positioned on top of plunger 12. A layer of the finely-divided refractory material 2 is then placed in cavity 9, the pellet 3 of electron-emissive material placed on top of this layer and the cavity is completely filled with more of the material 2. In some cases -I may place the pellet 3 directly on the disc 5 and fill the remainder of the cavity with the tungsten powder 2, i.e., so the electron emissive material will be partly embedded in the refractory material. In the present instance I use as the material 2 a tungsten powder which might be considered a coarse powder and which has the following particle size distribution:

Diameter,

Fraction Percent MMUIGI As shown in FIGURE 5 a suitably-supported block 14 is then placed on top of the mold and plunger 12 is moved upwardly into the position shown in FIGURE 5 whereby a high pressure is exerted on the composite mass 23. I have used a pressure of about 80,000 lbs. per sq. in. although other high pressures may be used. If lower pressures, for instance 15,000 lbs. per sq. in. are used with the above tungsten powder, the composite pressed body was very soft and porous and the activation time of the cathode was almost instantaneous with the result that there was excessive barium evaporation. On the other hand, use of higher pressures, for instance about 130,000 lbs. per sq. in. with the above tungsten powder resulted in a strong and durable cathode but the activation time was materially increased. Although the use of extremely high pressures, for instance about 200,000 lbs. per sq. in. increased the activation time, it made it possible to machine the cathode surface to a mirror-like finish which is very advantageous in some instances. In general it is possible to vary the particle size, pressure and temperature while obtaining suitable porosity, proper adherence of the particles to each other and to the retaining member, thus securing 'a cathode which is mechanically strong, has high emission and also has a long life.

Due to the high pressures used the disc 5 is compressed and forced out against the member 4. As a result of this and the adherence of the particles of the tungsten, the disc 5 is firmly held in position. 7

The cathode is then removed from the mold and heated in vacuum at a temperature of about 1270 to 1300 for about 10 to 20 minutes. The cathode is preferably heated at a low rate of increase to a temperature of 127 0 C., for instance in about 10 minutes, to ensure that the carbonates will break down to the oxides at a low rate. In some cases this heating may be effected after the cathode has been mounted in a tube, for instance during the activation of the cathode, but I prefer to heat it prior thereto in order to avoid possibility of damage due to handling. Instead of heating in vacuum I may heat in other inert atmospheres such as hydrogen, helium, argon or neon.

I have found that cathodes made by my method are firmly held in the retainer ring and the emitting surface can be machined to a smooth finish without unduly increasing the activation time. The activation time of the cathode made by the above method was about 30 to 45 minutes at a temperature of 1250" C.

The cathode shown in FIG, 3 is formed in the manner described above except for the formation of the nonporous portion 7. In making this cathode first provide on the surface of disc 5 of FIG. 4 a layer of finelydivided refractory material which may be of a particle size much less than that of the remaining material so that it forms a non-porous layer after being subjected to heat and pressure. However, I prefer to use a rathercoarse powder and much greater pressures, i.e., 200,000 to 400,000 lbs. per sq. in., as this increases the mechanical strength of the portion 7'. The cathode is then completed in the manner described above.

It will be noted that in my cathode the pores of the refractory material form the largest passageways connecting the electron emissive material to the outside of the cathode. When the emissive material is completely embedded in the porous refractory material, the pores of the material do, of course, form the sole passageways for the egress of the electron emissive material. When the pellet of electron emissive material is only partly embedded in the porous refractory material, for instance when it rests directly upon the surface of disc 5, the adherence of the refractory material to the disc is so good as to prevent undesired egress of electron emissive material along the surface of the disc and there is also a very good pressure seal formed between the periphery of disc 5 and member 4.

While I have described my invention in connection with specific constructions and examples, I do notdesire to be limited thereto as equivalent modifications will readily present themselves to one skilled in this art.

What I claim is:

1. A method of making a cathode of the dispenser type comprising the steps of forming within a retaining member a composite mass comprising a pellet of electron emissive alkaline earth material embedded in a mass of finely-divided refractory metal, subjecting the composite mass to high pressure to form the same into a compacted mass in which the compacted refractory material has a porous portion between the electron emissive material and an exposed surface of the mass and heating the compacted composite mass while within the retainer to cause the particles of the refractory material to adhere to each other and to the surface of the retainer.

2. A method of making a cathode of the dispenser type comprising the steps of forming within a cup-shaped retaining member a composite mass comprising a pellet of electron emissive alkaline earth material embedded in a mass of finely-divided refractory metal, subjecting the composite mass to high pressure to form the same into a compacted mass in which 'the compacted refractory material has a porous portion between the electron emissive material and an exposed surface of the mass and heating the compacted composite mass while within the retainer to cause the particles of the refractory material to adhere to each other and to the surface of the retainer.

3. A method of making a cathode of the dispenser type comprising the steps of forming within a retaining memher a composite mass comprising a pellet of electron emissive alkaline earth material completely embedded in a mass of finely-divided refractory metal, subjecting the composite mass to high pressure to form the same into a compacted mass in which the compacted refractory material has a porous portion between the electron emissive material and an exposed surface of the mass and heating the compacted composite mass while within the retainer to cause the particles of the refractory material to adhere to each other and to the surface of the retainer.

4. A method of making a cathode of the dispenser type comprising the steps of forming a non-porous layer of refractory material within a retaining member, forming on top of said layer a composite mass comprising a pellet of electron emissive alkaline earth material, embedded in a mass of finely-divided refractory metal, subjecting the composite mass to high pressure to form the same into a compacted mass in which the compacted refractory material has a porous portion between the electron emissive material and an exposed surface of the mass and heating the compacted composite mass while within the retainer to cause the particles of the refractory material to adhere to each other and to the surface of the retainer.

5. A method of making a cathode of the dispenser type comprising the steps of forming within a retaining member a composite mass comprising a pellet of electron emissive alkaline earth material embedded in a mass of finely-divided refractory metal, subjecting the composite mass to a pressure of about 15,000 to 400,000 lbs. per sq. in. to form the same into a compacted mass in which the compacted refractory material has a porous portion between the electron emissive material and an exposed surface of the mass and heating the compacted composite mass While within the retainer at a temperature higher than about 1250 C. and not more than about 1300 C.

to thereby cause the particles of the refractory material to adhere to each other and to the surface of the retainer. 6. A method of making a cathode of the dispenser type comprising the steps of forming within a retaining member a composite mass comprising a pellet of electron emissive alkaline earth material embedded in a mass of finelydivided refractory metal, subjecting the composite mass to high pressure to form the same into a compacted mass in which the compacted refractory material has a porous portion between the electron emissive material and an exposed surface of the mass and heating the compacted composite mass during the activation of the cathode to cause the particles of the refractory material to adhere to each other and to the surface of the retainer.

7. A method of making a cathode of the dispenser typ comprising the steps of forming within a cup-shaped molybdenum retaining member a composite mass comprising a pellet of electron emissive alkaline earth material embedded in a mass of finely-divided tungsten subjecting the composite mass to pressures of about 15,000 to 400,000 lbs. per sq. in. to form the same into a compacted mass in which the compacted refractory material has a porous portion between the electron emissive material and an exposed surface of the mass and heating the compacted composite mass at a temperature above about 1250 C. and not more than 1300 C. to cause the particles of the refractory material to adhere to each ,7

other and to the surface of the retainer.

References Cited in the'file of this patent UNITED STATES PATENTS 

1. A METHOD OF MAKING A CATHODE OF THE DISPENSER TYPE COMPRISING THE STEPS OF FORMING WITHIN A RETAINING MEMBER A COMPOSITE MASS COMPRISING A PELLET OF ELECTRON MEMBER A COMPOSITION MASS COMPRISING A PELLET OF ELECTRON EMISSIVE ALKALINE EARTH MATERIAL EMBEDDED IN A MASS OF FINELY-DIVIDED REFRACTORY METAL, SUBJECTING THE COMPOSITE MASS TO HIGH PRESSURE TO FORM THE SAME INTO COMPACTED MASS TO HIGH PRESSURE TO FORM THE SAME INTO A COMPACTED MASS IN WHICH THE COMPACTED REFRACTORY MATERIAL HAS A POROUS PORTION BETWEEN THE ELECTRON EMMISSIVE MATERIAL AND POROUS PORTION BETWEEN THE ELECTRON EMISSIVE MATERIAL AND AN EXPOSED SURFACE OF THE MASS AND HEATING THE COMPACTED COMPOSITION MASS WHILE WITHIN THE RETAINER TO CAUSE PACTED COMPOSITE MASS WHILE WITHIN THE RETAINER TO CAUSE THE PARTICLES OF THE REFRACTORY MATERIAL TO ADHERE TO EACH OTHER AND TO THE SURFACE OF THE REATAINER. OTHER AND TO THE SURFACE OF THE RETAINER. 